An Upper House inquiry has been told confusion over whether an email was the same as a paper document may have been the reason it did not receive most of the material related to the Mt Penny coalmine.

Last month the Independent Commission Against Corruption recommended criminal charges be pursued against former Labor government frontbenchers Ian Macdonald and Eddie Obeid over the granting of an exploration licence for the Mt Penny mine.

A wealth of material was tendered to the ICAC and the Legislative Council is examining why the same number of documents were not produced when it issued a call for papers on the same topic in 2009.

The head of the Trade department Mark Paterson believes there is a simple explanation.

He commissioned an external review which has concluded that staff, given just 24 hours to respond to the order, made a basic error.

"They did not search email records because there was a perception that it was a call for papers," he said.

He has told the inquiry he is satisfied there was no improper withholding of material.